1. Field of the Invention
The invention relates to thermoplastic molding compositions of an aromatic polycarbonate and a styrene-acrylonitrile copolymer.
2. Brief Description of the Prior Art
Blended compositions of a polycarbonate and a thermoplastic styrene-acrylonitrile copolymer have been described in the prior art literature; see for example the description in the U.S. Pat. No. 4,172,103. However, these blends as molding compositions produce articles of relatively poor elastic modulus. It is theorized that the inferior elastic modulus may be due to a phase incompatibility of the component resins. Certainly it is well known that blends of styrene-acrylonitrile and polycarbonate resins are not miscible as evidenced by their exhibiting two glass transition temperatures; see J. Appl. Polymer Sci. 29, 3131 (1984).
When blends of two different polymers exhibit a single glass transition temperature (T.sub.g), it generally signifies the resin components are compatible. However, a single T.sub.g is not a necessary condition for a compatible blend.
It was previously known that blends of styrene-acrylonitrile copolymers in admixture with polysulfone-carbonate resins exhibited a single T.sub.g and were miscible polymers; see U.S. Pat. No. 4,511,693. However, it was not previously appreciated that the polysulfone-carbonate copolymers functioned in some way to render miscible, blends of otherwise immiscible polymer resins.
When used herein and in the appended claims, "miscibility" refers to blends or compositions of polymers in which the component polymers do not undergo phase separation, thus helping to avoid stratification of the components during or after processing. "Miscibility" is of great significance for an admixture of different resins, because it ensures homogeneity, and a unity of properties, which greatly facilitates subsequent processing and use of the composition. Incompatible blends separate into phases containing predominantly their own separate components, and thus may be considered to be immiscible. This characteristic, combined with the often low physical attraction forces across the phase boundaries, usually causes immiscible/incompatible blend systems to have poor mechanical properties, thus preventing the preparation of polymer blends useful for a number of applications.